What is a rat bike?

Rat bikes are motorcycles that have fallen apart over time but been kept on the road and maintained for little or no cost by employing kludge fixes.

Say your bike needed an exhaust, instead of buying an exhaust that fits the bike you would use the very cheapest or free exhaust you could find, wether it was for your bike, or any other bike, or a car, or absolutely any exhaust that would pass the next MOT with it fitted, and then attach it to the bike using the cheapest or free method available.

Survival bikes look similar to but are different in purpose from rat bikes; they are modified for stylistic reasons.